This very much depends on the circumstances of your individual relationship with your agent/principal.  If you are an agent, it may be advantageous to you not to have a written agreement so that you are not bound by sales targets and so that you may benefit from the payment of compensation on termination under Regulation 17.  While there are definite advantages in having a written agreement, if the written agreement is poor or is drafted without an understanding of the workings of the Regulations, it may not do its intended job and may fail to provide you with the protection which you were expecting it to.